My theory based on what we saw until now:
The master and the head of a rightous clan were a pair and defeated the blood clan. But afterwards, he left the master and married a woman who gave birth to "Woo".
I suspect Kang-Oh to be the master's son who somehow ended up as a servant to "Woo" when he was a kid. For reasons unknown, "Woo" was in great danger and Kang-Oh somehow saved his life by thinking of trading his own. But both survived and "Woo" begged master to save Kang-Oh by probably giving up his martial arts and getting crippled as the price. As Kang-Oh lost his memory, he perceives master as his only saviour. Maybe master is afraid that, when Kang-Oh finds out about the sacrifices "Woo" made, he will turn his back at him and leave him as well.
I don't know the chapters after this nor do I know the story. It's just my interpretation of what was said until now. So I can be completely wrong.